1. Field of the Invention
This invention relates generally to the regeneration of anion exchange resins sorbed with perchlorate. More particularly, the invention applies to a combination of electrocatalysis and chemical reduction of perchlorate sorbed on a resin. While still sorbed to a resin, perchlorate anion is reduced to chloride ion that is easily eluted from the resin. In the process, the reduced forms of complexed metals such as Ti(III), V(II), V(III), and rhenium(V) [as Re(CH3)O2] are oxidized to Ti(IV), V(III), V(IV), and Re(VII) species. These oxidized species are then regenerated by electrochemical reduction and recycled through the resin bed for the further reduction of perchlorate. Complexing agents such as oxalate may be used to complex the reduced and/or oxidized metal species, and organic solvents such as ethanol may be used to enhance the reduction kinetics of perchlorate by Ti(III) species. The chloride ion may be left on the resin or the resin may be treated further to accomodate the desired degree of regeneration required for multiple reuse.
2. Background of the Art
Groundwater or surface water contamination with perchlorate (ClO4xe2x88x92) is known at numerous locations in the United States and abroad as described by Damian (Environmental Protection June 1997, p. 24). The perchlorate anion generally originates as a synthetic contaminant in the environment, primarily from the disposal of solid salts of ammonium or sodium perchlorate used in the explosives and rocket propellant industries. Under both oxygenated and suboxygenated environments perchlorate salts are in the dissociated form of anions, which are highly soluble and mobile in the subsurface aquifer. They may persist for many decades under typical groundwater and surface-water conditions because of perchlorate""s low reactivity with other organic or inorganic constituents. Because perchlorate anions are nonvolatile and highly soluble in water, they cannot be removed from water by conventional filtration, sedimentation, or air-stripping methodologies. Selective anion exchange resins (customized resins) are among the preferred treatment technologies for removal of perchlorate because the treatment method is highly efficient and capable of removing these anions to low levels in the presence of high levels of more commonly encountered monovalent anions.
However, the treatment of perchlorate by ion exchange also presents a challenge because of the difficulty in regenerating anion exchange resins saturated with these anions. The increasing order of affinity of singly charged ions for Type I anion exchange resins is well known: bicarbonate less than chloride less than nitrate less than perchlorate. The unusually high affinity between resins and perchlorate requires the use of large quantities of brine solution for the regeneration (Guter, Extended Abstract 218th ACS National Meeting, Vol. 39, No. 2. Pg 76, New Orleans 1999). This translates into high operating cost and environmental waste disposal problems. For example, Tripp and Clifford reported (ibid, 79) that, even with a relatively non-selective anion exchange resin and with counterflow regeneration, it required a large excess of sodium chloride for regeneration: 10 equivalents of chloride for each equivalent of resin exchange sites. With a relatively selective anion exchange resin, Batista et al. (ibid, 84) found that many bed volumes of 12% sodium chloride removed only about 6% of the loaded perchlorate from such a resin. Heating perchlorate-laden strong-base resins during regeneration results in only a small improvement in performance. It was therefore predicted that high operating costs for resin regeneration and regenerant disposal render this ion-exchange technology unattractive unless the two issues of resin regeneration and final waste disposal are resolved.
Highly selective anion exchange resins offer some advantages over conventional nonselective resins in the treatment of perchlorate-contaminated water because of their relatively high efficiency and capacity for the perchlorate species. For example, Oak Ridge National Laboratory has recently developed a new class of bifiuctional anion exchange resins, which are highly selective and efficient for the removal of perchlorate and pertechnetate from contaminated water in the presence of more ubiquitous but less toxic anions such as chloride, nitrate, and bicarbonate (U.S. patent application Ser. No. 08/212,198). However, because perchlorate anions are so strongly sorbed to the bifunctional resins, the conventional regeneration technique by washing with a brine (e.g., 12% sodium chloride) is ineffective.
Because most of these synthetic resins are expensive with a current market price of $400 to $1000 per cubic foot, the resin itself contributes to a major capital cost for the application of ion-exchange technology to remove perchlorate from contaminated water or other liquid streams. Cost-effective regeneration is necessary. U.S. patent application Ser. No. 09/491,242, discloses customized regenerants and sequences of addition addressing this problem.
The redox reaction of Ti(III) aquo ion and perchlorate is known and the kinetics have been reported by Cope et al., J. Chem. Soc. A; 301 (1967). Reduction of perchlorate by the complexed Ti(III) species Ti(III) (HEDTA) was reported by Liu et al., Inorg. Chem. 23, 3418 (1984). Both papers reported that chloride ion was the ultimate reduced species, which is nontoxic. However, no studies or reports have been published for the reactions between Ti(III) and perchlorate that was sorbed on the resin, nor the methodology for regenerating the anion-exchange resins by reduction with Ti(III).
The oxidized Ti(IV) species (by reduction of perchlorate) are readily reduced back to Ti(III) species using an electrochemical process as described in, inter alia, U.S. Pat. No. 5,250,162 to Foller et al. However, this electrochemical reduction process has not been applied for the regeneration of anion exchange resins. In particular, organic complexing agents such as oxalate must be utilized in order to prevent the oxidative precipitation of Ti(IV) species within the resin bed.
Abu-Omar et al., Inorg Chem. 35, 7751 (1996) described the kinetics of the reduction of perchlorate by methylrhenium dioxide and ascribed the high reactivity of this compound as a reducing agent to its oxophilicity, the stability of the resultant trioxide and the low coordination number of the dioxide. Energetically, methylrhenium dioxide appears to have favorable properties for reduction of perchlorate but practical application has not been reported.
It is an object of this invention to economically regenerate anion exchange resins sorbed with perchlorate. It is a further object of this invention to regenerate perchlorate-loaded anion exchange resins while producing a minimum volume of waste for further treatment. It is yet an additional objective of this invention to regenerate perchlorate-loaded anion exchange resins using a minimal amount of equipment and energy. These and other objects of the invention may be achieved by a combination of chemical reduction of perchlorate to chloride species on the exchange resin by Ti(III) species and electrochemical reduction of Ti(IV) to Ti(III) species in a continuous process.
We have discovered a new method and an electrochemical process for regenerating anion exchange resins loaded with perchlorate. In one aspect of the invention, we use Ti(III) complexed with a water solubilizing chelating agent such as oxalate to reduce perchlorate according to the equation.
8[Ti(III)xe2x88x92(Ox)2xe2x88x92(H2O)2]xe2x88x92+(ClO4xe2x88x92)sxe2x86x928[Ti(IV)xe2x88x92(Ox)2xe2x88x92(H2O)2]+Clxe2x88x92xe2x80x83xe2x80x83(1)
Ethyl alcohol in the range of 0 to 90% may be added to enhance the reduction kinetics of perchlorate by the Ti(III)-oxalate complexes. In another aspect of the invention, we use the neutral compound Re(CH3)O2 as a reductant to reduce perchlorate to chloride ions to regenerate the resin. In the first embodiment, the eluate is then passed through an electrochemical cell to reduce the Ti(IV) back to Ti(III) and the eluant is recycled. A relatively small quantity of regenerant is therefore required by this regeneration process because of a continued regeneration and recycle of the Ti(III) reductant through the system. In the case of methylrhenium oxide, the V(II)/V(III)/V(IV) couples are used as catalysts for regeneration of the Re(CH3)O2/Re(CH3)O3 pair at an electrode. The new regeneration process is efficient and cost-effective, while not being subject to difficult-to-maintain operating conditions, nor generating large quantities of secondary wastes.
This invention uses two preferred reductants, complexes of Ti(III)-oxalate and/or the neutral compound Re(CH3)O2 to reduce perchlorate, while still sorbed to the resin, to chloride ion that is easily eluted from the resin. After a complete reduction of perchlorate, a second regenerant consisting of a dilute acid and sodium chloride is used to elute excess amount of Ti-oxalate or methylrhenium oxide complexes from the resin thereby regenerating the resin. The practice of the invention, therefore, provides a method and an electrochemical reduction process for continuous regenerating the anion exchange resins either in a fixed bed, continuous alternating column, or in a batch process that may be used for removing perchlorate or some other specific anions such as periodate and nitrate from water or other liquid streams. This new regeneration method for anion exchange resins offers an improved regeneration efficiency and waste minimization compared with the conventional chemical displacement technique using sodium chloride brine.